At some point in their life about 10% of people will get kidney stones, a painful condition in which calcium deposits clog the kidneys.

Scientists aren't sure what causes these deposits. But one theory that Lieske and colleagues are investigating is that tiny calcium-covered particles are partly to blame.

Previous research has found such particles in human serum, urine, renal cysts from patients with kidney disease, as well as in kidney stones.

Lieske says some researchers dub the particles nanobacteria, and propose they are a new disease-causing agent.

But Lieske says there is not yet enough evidence these particles are alive.

Nanoparticle investigation

Lieske's team isolated the nanoparticles, which have a protein-lipid core surrounded by a calcium phosphate shell, from kidney stones.

The researchers grew the nanoparticles in culture over a period of four to eight weeks.

They found that antibiotics and metabolic inhibitors slowed the particles' growth.

Then the researchers grew a large batch of nanoparticles, dissolved the calcium shells and extracted proteins and DNA.

Does all this mean that the nanoparticles are nanobacteria after all?

Lieske says it's still too early to say.

"There definitely is DNA associated with [the nanoparticles]," he says. "But is that a contaminant?"

He says some fragments of the protein and DNA appear to match known bacteria.

His team now plans to grow more nanoparticles and see if they can find a unique genetic signature that would prove the nanoparticles are indeed nanobacteria.

Scepticism

Geologists and astrobiologists have also considered the possibility of nanobacteria over the years. For instance, understanding the full range of life forms is important in our search for extraterrestrial life.

They concluded, he says, that anything smaller than 100 nanometres could be capable of independent life.

"This latest report is interesting but I notice how cautious [the authors] are in what they say," says Walter.

Chemical self-assembly

Whether they are alive or not, understanding the role of nanoparticles in kidney stones will be useful in developing treatments, says Lieske.

The particles could still be infectious disease-causing agents whose chemistry allows them to self-assemble.

"That could be a very interesting pathogenic process that might be something like prion proteins where it is disease causing but it's not necessarily an organism like we think of traditionally," says Lieske.

Either way, Lieske and team are containing the nanoparticle experiments so the particles don't accidentally give anyone a disease.